Multimode horn antenna

ABSTRACT

A conical or pyramidal horn-radiator having small flare angle changes within the horn at cross-section dimensions much larger than the input waveguide, to excite higher order modes which control the E-plane aperture distribution and produce a tapered aperture field in the E-plane. Equal E- and H-plane beamwidths are thereby obtained. The horn has particular use as a feed for a parabolic reflector, such as a Cassegrain antenna. The flare angle changes are used for pattern improvement in a first frequency band, and separate means are provided for pattern improvement in a second band.

I United States Patent [151 3,662,393 Cohn 1 May 9, 1972 54] MULTIMODEHORN ANTENNA 3,530,481 9 1970 Tanaka et a1. ..343/786 72 1 t Se B. nvenor ymour Cohn, Tarzana, Calif Primary Emminer Eli Lieberman [73]Assignee: Emerson Electric Co., St. Louis, Mo. Attorney-Folster andPolster 22 F'] d: Feb. 20 1970 l 1 e 57 ABSTRACT [21] Appl. No.: 12,970I A conical or pyramidal horn-radiator having small flare angle changeswithin the horn at cross-section dimensions much [52] Cl "343/786 largerthan the input waveguide, to excite higher order modes [5 l 1 f- 9 13/00which control the E-plane aperture distribution and produce a [58] Fleldof Search ..343/772, 783, 786 tapered aperture field in the Equal and Hp|ane beamwidths are thereby obtained. The horn has particular use [56]Rderences Cned as a feed for a parabolic reflector, such as a Cassegrainanten- UNITED STATES PATENTS na. The flare angle changes are used forpattern improvement in a first frequency band, and separate means areprovided for 3,4 l l COQk attern improvement in a econd bancL 3,482,25212/1969 Nagelberg.. .....343/786 3,510,875 5/1970 Beguin ..343/786 10Claims, 9 Drawing Figures PATENTEDMM 9 I972 SHEET 1 UF 3 5EYMQUR 5. COMMPATENYEDMY 9 I972 SHEET 3 BF 3 5.7 GHz E-PLANE H-PLANE 6m 5EYM9UI2 B.COHM Q1 9% wii dZZW a A MULTIMODE HORN ANTENNA BACKGROUND OF THEINVENTION This invention relates to an improved horn design and a methodfor improving the E-plane aperture distribution of a pyramidal orconical horn (i.e. a horn having,.respectively, a square or a circularcross-section throughout its length). For convenience, a horn of eitherof these configurations will be referred to herein as a horn of regularcross-section. It has particular, but not exclusive, use as a feed hornfor a parabolic reflector.

It has long been sought, in the use of both pyramidal and conical feedhorns for radiating electromagnetic energy from a waveguide into freespace, to produce beams having low sidelobes and equal E- and I-I-planebeamwidths. A number of proposals have been made for producing thesedesirable characteristics in a horn, many of which have been rathersuccessful. All of these approaches, however, have had certaindrawbacks, such as limitation to narrow bandwidths, high dissipation orreflection losses, low power capabilities, limitation to particularpolarizations, cost of fabrication, or complexity.

One of the objects of this invention is to provide'a horn and a methodfor reducing the sidelobes and equalizing the E- and I-I-planebeamwidths of a transmitted beam.

Another object is to produce such a horn which is of regularcross-section, thereby permitting circular or any linear polarization.

Still another object is to provide such a horn which has a simple, cleanconstruction providing economical fabrication, high power capabilities,minimal dissipation loss, and low VSWR.

Other objects will become apparent to those skilled in the art in thelight of the following description and accompanying drawings.

SUMMARY OF THE INVENTION In accordance with this invention, generallystated, a horn and method are provided for producing a desired taperedaperture field in the E-plane in a horn of regular cross-section, byinducing higher order modes in the horn by means of small changes offlare angle at one or more points within the horn. The changes of flareangle are at large cross-sectional dimensions of the horn. The termsmall" as applied to the flare angle changes is used herein to indicatea change of a few degrees, up to approximately The change may be eitherpositive or negative, as will be explained more fully hereinafter. Theterm large as applied to the dimensions at which the changes occurindicates a width of at least three or four times that of the smallestwaveguide, of the same regular cross-section as the horn, which willsupport a dominant mode of a frequency band to be affected by the horn.Otherwise stated, although widths larger than the free space wavelengthsare required to propagate TE or TM, modes respectively in square orcircular cross-sections, the means known heretofore have involved theuse of rather severe discontinuities, such as steps or large flare anglechanges, for inducing higher order modes at dimensions of less than 1.5wavelengths. The discontinuities of this invention are at dimensionswhich are substantially greater than 1.5 wavelengths. Flare anglechanges at smaller dimensions may also be utilized, but the major partof the higher order mode energy is induced by the flare angle changes atlarge cross-sectional dimensions.

For convenience, the operation of the horn of this invention isdescribed in terms of transmitting wave energy from a transmission linefeeding the throat of the horn, through the horn and out of the homsaperture. It will be understood, however, that the operation of the hornmay be, and generally will be, reciprocal. Therefore, in a frequencyband for which the flare changes are designed to be effective, the hornwill provide the same advantages in either a transmit or receivefunction. In fact, as will be described more fully hereinafter, in thepreferred embodiment the flared sections are proportioned to have aminimal cumulative effect on wave energy in a transmit band, and areeffective only on wave energy in a lower frequency receive band.Separate means are provided in the throat of the horn for stimulatinghigher order modes in the transmit band.

In a preferred form of the invention, the horn is a feed born for aCassegrain type antenna, fed at its throat through a waveguide of thesame regular cross-section as the horn. Also in the preferredembodiment, two approximately equal flare angle changes are provided inat least one part of the horn, and the length of the section between thechanges is proportioned to cancel yet higher modes than the dominantmode supported by the waveguideor a first higher order mode induced bythe flare angle changes. Also in the preferred embodiment, the regularcross-section of the horn is square, the dominant mode in the waveguideis TE and the first higher order mode induced by the flare angle changesis TEfIM The term TE/TM denotes a linear superposition of TE and TM,mode amplitudes in such amplitude relationship that cross-polarizedE-field components are cancelled.

For small flare angle changes, the efiect of multiple changes isadditive. Therefore, multiple flare angle changes may be utilized toproduce sufficient amplitude of the higherorder mode for equalizing theE- and H-plane beamwidths, for example.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings,

FIG. 1 is a view in side elevation of one illustrative embodiment ofantenna of this invention;

FIG. 2 is a diagrammatic detail in section showing theoretical phasefront relationships at a flare angle change (involving a decrease inflare angle) of a horn part of an antenna of this invention;

FIG. 3 is a diagrammatic sectional view corresponding to FIG. 2, showingtheoretical phase front relationships at an increasing flare anglechange;

FIG. 4 is a detail illustrating one aspect of the present invention;

FIG. 5 is a detail in section of a waveguide part of the antenna shownin FIG. 1;

FIG. 6 is a diagrammatic representation of the antenna of FIG. 1 as acascaded directional coupler; and

FIGS. 7a-7c show a set of graphs showing receive band E- and H-planeaperture distribution.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,and particularly to FIG. 1, reference numeral 1 indicates oneillustrative embodiment of means of this invention for transmitting orreceiving a beam of electromagnetic wave energy, i.e. an antenna. Theantenna 1 includes an input transmission line part 3 in the form of awaveguide 31 and a horn part 5. The waveguide 31 is square incross-section and is proportioned to support electromagnetic wavepropagation inthe TE mode in both a 3.7 to 4.2 GHz receive band and a5.925 to 6.425 Gl-Iz transmit band. These frequencies correspond towavelengths on the order of 3 and 2 inches respectively. Means 35 areprovided in the waveguide for inducing higher order modes in thetransmit band. The waveguide 31 is proportioned to support adominant'mode in both the receive and transmit band, but to support thehigher mode induced by the means 35 only in the higher frequencytransmit band.

The horn part 5 is pyramidal, that is it is square in cross-sectionthroughout its length. The horn part 5 consists of a series of sections51, 53, 55, 57 and 59, joined to each other in end to end relationshipto form flare angle changes 61, 63, 65 and 67 respectively. The inputend of the first section 51 is joined to the output end of the waveguidepart 3 to form a flare angle change 33. The output end of the waveguideand of each section matches the input dimension of the next successivesection, so that the flare angle changes form discontinuities, but notsteps, in the horn part. Each section has a flare angle 0 which may bedefined as the angle formed by the sides of the section with a centralaxis of symmetry of the antenna. The first flared section 51 has aninput dimension a and a positive flare angle 0,, the second flaredsection 53 has an input dimension a, and a flare angle 6,, less than 0,;the third section 55 is straight, and has a cross-sectional dimension aand a flare angle 0,, equaling zero; the third flared section 57 has aninput dimension a equal to a; and a positive flare angle and the outputflared section 59 has an input dimension a and a positive'flare angle 0greater than 6 The output flared section 59 has an output dimension,i.e. an antenna aperture 52 dimension, of (1,. The sections between thefirst flared section 51 and the output flared section 59 may beconsidered as connecting means for these two flared sections.

The antenna 1 forms a part of an antenna system, which includes signalgenerating and receiving means, not shown. The antenna of FIG. 1 isparticularly adapted for use in a Cassegrain antenna with a 97 footdiameter parabolic reflector for communication satellite systems.Cassegrain antenna systems are well known and are not here illustrated.Such a system also requires a transmit-receive junction, circularpolarizer and difference-pattem couplers, all of which may be ofstandard construction and are not shown.

The details of the structure of the antenna 1 may best be understoodafter a brief discussion of an approximate analysis of the principlesused in designing such a device.

The use of flare angle changes to induce higher modes for aperturepattern improvement has been described heretofore, but application ofthis technique has been limited both by lack of understanding of theprinciples involved and by a belief that changes should be at thesmallest possible dimensions, to suppress higher order modes. As aresult, the design of such devices has been confined to rudimentary andinflexible designs useful only in narrow bandwidths and for limitedpurposes. They have therefore been impractical for most uses, especiallyuses requiring both transmit and receive functions to be performed bythe same horn.

The following analysis of the effects of flare angle changes and sectionlengths on relative phase amplitudes, relative phase angles and aperturedistribution involves a number of approximations, but is accurate enoughto provide formulas adequate for design purposes.

In FIG. 2 a section 71 of flare angle 0,, is joined to a section 73 offlare angle 0,, 0. Phase fronts d and e apply to the two horn sections71 and 73 respectively and intersect at the flare angle discontinuitypoint 75 at cross-sectional diameter a. The maximum separation betweenthese phase fronts is Az R L 11/2 [cscO cot0,,] a/2 tan 0 /2 (I) If 0,,is less than about 0.4 radians (23), A: may be approximated by Az= 5 (0in radians) Similarly, if 0,, and 0, both differ from zero, as shown inFIG. 3, the phase front separation is Az n s) The TE mode phasedifference corresponding to Az is 2 1rAz/A,, or

1ra m-x-g a" b) where A, is guide wavelength of the TE horn mode and ais the E-plane height at the flare change point 75. For 0,, and 0,small, and an H-plane width greater than about two wavelengths, A isapproximately equal to A.

Assume for now that the horn is sectoral with E-plane flare and constantH-plane width. Then with a small flare angle change, an incident TE,wave arriving from the left yields predominately TE on the right plussmall amplitudes of TIE/TM TE/TM etc. Reflected waves in the variousmodes are relatively much smaller and will be ignored. In FIG. 2 the yaxis lies in a phase front of the 6,, 0 horn section 73. This case issufficiently general for analysis, since equation 4 shows where Arepresents complex amplitudes of the TE TE/TM TE/T M etc., modes. To afirst order with A3, A A equation 5 can be satisfied by en 2 :A cos n=17 Thus, the TIE/TM TE/TM components are imaginary and are in phasequadrature with respect to the transmitted TE component.

Equation (6) is a Fourier series in the period y a/2 to a/2. Tosimplify'the result, let y) be approximated by a half sine wave. Thenthe Fourier coefficients yield the following TIE/TM mode amplitudes forthe TE mode:

11 :1; I 2=j g $=j W (by Equation (4)); A4= A2;

These amplitudes apply in the region just to the right of thediscontinuity.

If a pyramidal rather than sectoral horn in considered, the phase frontswill be spherical; hence T5 TE TIE/TM etc., modes will be needed tosatisfy E,,(x) sin (rrx/a) and d: (x). However, the half-sine-wavevariation of E,,(x) results in extremely r id convergence of the Fourierexpansion for sin (7rx/a)] and modes of higher order than unity in theH-plane may be neglected in practical design.

The formula for the amplitude A, of the TE/TM mode as given by equation7 may be used to design the flare angle change that yields a desiredratio of TE/TM amplitude to TE amplitude. Note that the phase angle ofA, lags .4 by when 0,, 0,, 0. Therefore, if the length of horn betweenthe flare change and the aperture has 90 differential phase shiftbetween the TE and TE/TM modes, these modes will be in phase at thecenter of the aperture. Similarly, when 0, 0, 0, 270 differential phaseshift is required. Thus, the decrease in flare angle in FIG. 2 should befollowed by 270 differential phase shift, and the increase in flareangle in FIG. 3 by 90.

The phase shift (1, of the TE and TM,,, modes in a square pyramidal hornis given in radians by where a, is the aperture height and width at theleft; a is the same at the right, and 0 is the flare angle. This formulais an approximation valid for small and moderate flare angles. The axiallength is L=(a,a,/2)cot0. (9) When 0= 0 and a, (1,, 4a,, is simply Zn-LJ(A 1 1 n x 2a Equation 8 or 10 with n set equal to 0 and 2 is used tocalculate the differential phase shift d, for the TE and TE/T M modes.

The parameters a,, a 0 and L should be adjusted by successive trials toprovide the desired A values, with required 90 or 270 differential phaseshifts and reasonable dimensions. If sufficient A cannot be produced bya single flare change, two flare changes may be used. Small A values aredirectly additive to first order at the aperture. Frequency sensitivityis greater, however, than in the case of a single flare change at 90differential phase spacing from the aperture.

Equation 7 shows that the amplitudes A A are a rapidly decreasingprogression. Therefore, the presence in the aperture of components oforder greater than A will affect the radiation pattern only in a minorway. However, these mode amplitudes may be cancelled by the techniqueshown in FIG. 4 where the flare angle change 0,, 0,, is taken in twosteps 0,, 0,. and 0,. 0,, separated by an intermediate section 77 ofaxial length M. The length M is chosen so that the differential phaseshift 0,, 0 between the TE and the TE/T M modes is 180. To make theTE/TM amplitudes generated at each step equal, the sizes of the steps0,, 0,, and 0,. 0,, are chosen in such a way that the product of theflare angle change 0,, 0 times the cross-sectional dimension a at thefirst step is approximately equal to the product of 0 0,. times a, atthe second step. Because the dimension a is frequently about the samefor each step, the angle changes are generally about equal. Then thesmall TE/TM component excited at the first discontinuity will becanceled by that at the second.

In view of the foregoing theoretical considerations, the antenna of FIG.1 may be considered in more detail. The general approach in designingthis antenna was to provide two major flare angle changes connected byan adjustable straight section to provide additive conversion of TE modeto TE/TM mode. Each major flare change was then divided into twoapproximately equal flare changes for higher mode cancellation. Theexact proportions were then adjusted to provide optimum expectedperformance, and a final design was built and tested.

The flare angle 0 of the first flared section 51 is chosen to produce adesired amplitude of TE/T M mode at a reasonable cross-section a Thedimension a,, should be several times the width of the input waveguide31, and may be on the order of two-thirds of the desired outputaperture. The provision of the straight section 55 allows adjustment ofthe overall length of the horn, for experimental pattern improvement,and provides a substantial length for the TE/T M modes to phase properlywith the TB mode. The two flare angle changes between the section 55 and59 provide additional TE/TM, amplitude. The flare angle of the outputflared section 59 is determined by the difference between the dimensiona and the desired aperture dimension a and by the length/flare anglecombination which will provide approximately 90 differential phase shiftwith these parameters.

The flare angle change between the first flared section 51 and thestraight connecting section 55 is divided into two approximately equalflare angle changes at 61 and 63 by an intermediate section 53.Likewise, the flare angle change between the connecting section 55 andthe output flared section 59 is divided into two approximately equalflare angle changes at 65 and 67 by an intermediate flared section 57.As

has been discussed, the intermediate sections 53 and 57 are proportionedto cause a 180 differential phase shift in their length between TE modeand TE/TM modes at center bandwidth.

To a first approximation, the design criteria require a distanceproducing a 270 differential phase shift (between TE and TE/TM betweenthe center of section 53 and the aperture 52, and between the center ofsection 57 and the aperture 52. The amplitudes of the TE/T M modeexcited at each of these flare angle changes may be added, and thedesired cumulative amplitude relative to that of the T15 mode selected.A ratio of TE/T M to TE of 0.66 at the aperture would yield equal E- andH-plane beamwidths at the 10 db points. A ratio of 0.84 would yield bestsuppression of sidelobes. The illustrative embodiment was designed byapplication of the foregoing theory to provide a ratio near the first ofthese values, to produce equal beamwidths. The lengths were computed togive a good approximation to a plane phase front in the aperture and tosuppress TE/TM components.

Three factors need to be taken into account other than those alreadymentioned in designing a horn such as the illustrative embodiment shownin FIG. 1.

In calculating amplitudes of induced higher order modes, considerationmust be given to the fact that the use of successive approximately halfangle changes in the flare angle to cancel higher order modes causessome relative phase shift between the portion of the TE/l M mode energyconverted at the first flare angle change and that produced at thesecond flare angle change. In the illustrative embodiment, the length ofintermediate flared section typically yields a relative phase shiftbetween the TE mode and the TE/l' M mode of about 42; hence the totalamplitude of the two equal TE/TM modes is reduced by the factor cos(42/2) 0.934 at each end of the straight section 55 as compared with asingle flare change at each end of this section.

The second factor is that the flare angle of the output flared section59 produces a phase front curvature at the aperture. The phase curvatureerror is equivalent to a flare angle change from the output sectionflare angle to an imaginary 0 0 section at the aperture. The TE/TMcomponent produced by the flare angle change is in phase quadrature withthe incident TE component. The simplest way of correcting this effectand achieving an approximately planar phase front is by making thelength of the straight section 55 such that the TE/TM wave arriving atthe aperture has a component equal in amplitude and out of phase withthe component produced at the aperture.

The third factor is that consideration must'be made of not only thedecrease in TE mode at each successive flare angle change, but also theeffect of the presence of other components, particularly TE/TM in theincident wave. A far better quantitative analysis of the relative modeamplitudes, hence of aperture patterns, of a multiflare horn may beaccomplished by treating the flare angle changes as equivalent tocascaded directional couplers as shown in FIG. 6. Although thistechnique was not used in the design of the illustrative embodiment ofFIG. 1, it yields predicted values closely conforming to experimentallydetermined values for this embodiment. The formulas for directionalcouplers may be applied directly, with the ratio of input amplitude ofone mode to output amplitude of a second ==j( 2/ 3) 'a/A (0,, 0,)radians, and with taken from equations (8) or 10). It has been foundthat much more accurate results are obtained if the aperture 52 istreated as a flare angle change from the output section 59 to animaginary section having a flare angle 0.

The inducement of higher modes in the transmit (5.925 to 6.425 Gl-Iz)band may be accomplished by any of a number of well-known expedients,such as a discontinuity 35 positioned in the throat of the device (i.e.in the area of the junction of the waveguide 31 with the horn 5), insidethe waveguide 31 a short distance. The cut-off frequency of the TE/TMmode in the waveguide is 5.32 GI-Iz. Hence the discontinuity 35 and thefirst flare angle change 61 have no effect in the receive band, and maybe independently optimized for best amplitude and relative phasing inthe transmit band. Final experimental patterns in the transmit band aresimilar to those in FIG. 7b. As expected, the discontinuity 35 did notchange the receive band patterns. In the transmit band, the flare anglechanges of the illustrative embodiment of horn produce a set of TE/TMcomponents whose phasor sum computes to almost zero. This WIDTH (INCHES)a 2.80 a, 12.20 a, 14.00 a, 14.00 a, 15.38 a. 22.1 LENGTH (INCHES) 5131.8 53 13.25 55 50.18 57 16.96 59 42.5 FLARE ANGLE (DEGREES) 01 8.40 0,3.90 0

The antenna of FIG. 1 was built and tested. Experimental adjustment ofless than percent in the length of straight section 55 was required toproduce the best compromise patterns across the entire 3.7 4.2 GHZbandwidth. The E- and H- plane patterns obtained are shown for 3.7, 3.95and 4.2 GHZ respectively in FIGS. 7a, 7b and 70. As shown in FIG. 7, E-and H-plane patterns across the receive band show that the designachieves its intended objectives: the E- and H-plane beamwidths areclosely equal, the H-plane patterns are scarcely affected by flarechanges, and both the E- and I-I-plane sidelobes are highly suppressed.In comparison, the theoretical beamwidths of the H-plane and E-planepattern of an aperture containing a pure, plane TE field are in theratio of 1.35:1, while the theoretical sidelobe levels are 23 and 13 db,respectively. The maximum input VSWR measured 1.02 or less in both thereceive band and the transmit band.

Although a particular embodiment of the invention has been described, itwill be understood that numerous variations, within the scope of theappended claims, will be apparent to those skilled in the art in thelight of the foregoing description. For example, cancelling of higherorder modes may be accomplished by combinations of flare angle changesother than two sequential approximately equal changes spaced at 180diflerential phase shift. Negative, as well as positive, flare anglesmay be provided in the horn. The modes utilized may be other than thosedescribed. The means for inducing a higher order mode in the higherfrequency band may comprise one or more flare changes between thewaveguide and the first flared section, at a cross-sectional dimensionsmaller than the large dimensions at which the major part of the higherorder modes are induced in the lower frequency band. These variationsare merely illustrative. Having thus described the invention, what isclaimed and desired to be secured by Letters Patent is:

1. Means for transmitting or receiving a beam of electromagnetic waveenergy comprising:

an input transmission line having an output end, said transmission linecomprising at least one waveguide proportioned to propagateelectromagnetic wave energy in at least a first frequency band in adominant mode, and a horn of regular cross-section, said horncomprising:

a first flared section having a positive flare angle and having an inputend and an output end, said input end of said first flared sectioncommunicating with said output end of said input transmission line, saidoutput end of said first flared section having a large cross-sectionaldimension at least three times the cross-sectional dimension of theoutput end of the waveguide;

an output flared section having a positive flare angle and having aninput end and an output end, said output end of said output flaredsection comprising an aperture opening into free space; and

connecting means for connecting said output end of said first flaredsection with said input end of said output flared section, saidconnecting means forming a plurality of small angle changes within saidhorn including at least a first flare angle change with said firstflared section and an output flare angle change with said output flaredsection,

said flare angle changes being proportioned to convert a desired portionof electromagnetic wave energy transmitted from said input transmissionline through said horn in said first frequency band, from said dominantmode to at least one higher mode, and

said sections and said connecting means being proportioned such that intransmitting from said input transmission line through said horn in saidfirst frequency band, said higher mode wave energy produced at saidflare angle changes causes the horn to suppress sidelobes and to produceapproximately equal E- and H-plane beamwidths.

2. The means of claim 1 wherein said input transmission line is awaveguide of the same regular cross-section as said horn.

3. The means of claim 1 wherein said horn is square in cross-section,said dominant mode is TE,,,, and said one higher mode comprises TE/T M4. The means of claim 1 wherein said connecting means comprises a secondflared section connected to said first flared section and a thirdsection connected to said second flared section, the flare angle of saidsecond flared section being approximately half the sum of the flareangle of said first flared section and the flare angle of the thirdsection, said second flared section being so' proportioned that theflare angle change between said second flared section and said thirdsection cancels substantially all of a mode, different from saiddominant mode and said one higher mode, produced at the flare anglechange between said first flared section and said second flared section.

5. The means of claim 3 wherein said portion of TE/TM mode produced atsaid flare angle changes yields a ratio of TE/TM to TE of between about0.65 and about 0.84.

6. Means for transmitting and receiving electromagnetic wave energycomprising:

a waveguide part of uniform, regular cross-section, said waveguide partbeing proportioned to support a first mode in a transmit frequency bandand in a receive frequency band, and to support a second, higher, modein one only of said frequency bands,

a horn part of the same regular cross-section as said waveguide partconnected to said waveguide part,

means in said waveguide part for converting a desired portion of saidfirst mode of said one of said frequency bands to said higher mode, saidmeans in said waveguide part having substantially no effect on the otherof said frequency bands, and

means in said horn part for converting a desired portion of said firstmode of the other of said frequency bands to a second, higher, mode,said means in said horn part comprising a plurality of small flare anglechanges at large cross-sectional dimensions of said horn part at leastthree times the cross-sectional dimension of the waveguide part.

7. Antenna means for transmitting and receiving electromagnetic waveenergy comprising:

an input transmission line part having an output end, said transmissionline comprising at least one waveguide proportioned to propagateelectromagnetic wave energy in a dominant mode in at least a transmitfrequency band and in a receive frequency band, and

a horn part connected to said waveguide part at one end and having anaperture at its other end,

first means in said horn part for converting a desired portion secondmeans at the output end of the transmission line part for converting aportion of said first mode of the other of said frequency bands to saidhigher mode, said second means being located at a cross-sectionaldimension of said antenna means substantially smaller than said firstmeans and having substantially no effect on said one frequency band.

8. Means for transmitting or receiving a beam of electromagnetic waveenergy comprising:

an input transmission line comprising at least one waveguideproportioned to propagate electromagnetic wave energy in at least afirst frequency band in a dominant mode,

a horn part connected to said transmission line, said horn part havingat least four axial flare sections of different flaring angles, one ofsaid flare sections converting a desired portion of said dominant modeto a higher order mode and comprising a small flare angle change in saidhorn at a large cross-sectional dimension of said horn part at leastthree times the cross-sectional dimension of the input waveguide, saidhorn part being proportioned to produce a desired phase relationshipbetween said dominant mode and said higher order mode induced by saidflare angle change.

9. The means of claim 8 wherein said small flare angle change inducessaid first-mentioned higher order mode and a second higher order mode,and including means, comprising at least a second small flare anglechange, proportioned and spaced from said first-mentioned small flareangle change to cancel said second higher order mode.

10. Means for transmitting or receiving a beam of electromagnetic waveenergy comprising:

an input transmission line having an output end, said transmission linebeing proportioned to propagate electromagnetic wave energy in at leasta first frequency band in a dominant mode, and

a horn of regular cross-section, said horn comprising a first section; asecond, flared, section connected to said first section to form a firstflare angle change at a first crosssectional dimension of said horn; anda third section connected to said second section to form a second flareangle change at a second cross-sectional dimension of said horn, theproduct of said first flare angle change times said firstcross-sectional dimension being approximately equal to the product ofthe second flare angle change times said second cross-sectionaldimension, said first flare change inducing a first higher order modeand a second higher order mode in said first frequency band, said secondsection being so proportioned that said second flare angle changecancels substantially all of the second higher order mode in said firstfrequency band.

1. Means for transmitting or receiving a beam of electromagnetic waveenergy comprising: an input transmission line having an output end, saidtransmission line comprising at least one waveguide proportioned topropagate electromagnetic wave energy in at least a first frequency bandin a dominant mode, and a horn of regular cross-section, said horncomprising: a first flared section having a positive flare angle andhaving an input end and an output end, said input end of said firstflared section communicating with said output end of said inputtransmission line, said output end of said first flared section having alarge cross-sectional dimension at least three times the cross-sectionaldimension of the output end of the waveguide; an output flared sectionhaving a positive flare angle and having an input end and an output end,said output end of said output flared section comprising an apertureopening into free space; and connecting means for connecting said outputend of said first flared section with said input end of said outputflared section, said connecting means forming a plurality of small anglechanges within said horn including at least a first flare angle changewith said first flared section and an output flare angle change withsaid output flared section, said flare angle changes being proportionedto convert a desired portion of electromagnetic wave energy transmittedfrom said input transmission line through said horn in said firstfrequency band, from said dominant mode to at least one higher mode, andsaid sections and said connecting means being proportioned such that intransmitting from said input transmission line through said horn in saidfirst frequency band, said higher mode wave energy produced at saidflare angle changes causes the horn to suppress sidelobes and to produceapproximately equal E- and Hplane beamwidths.
 2. The means of claim 1wherein said input transmission line is a waveguide of the same regularcross-section as said horn.
 3. The means of claim 1 wherein said horn issquare in cross-section, said dominant mode is TE10, and said one highermode comprises TE/TM12.
 4. The means of claim 1 wherein said connectingmeans comprises a second flared section connected to said first flaredsection and a third section connected to said second flared section, theflare angle of said second flared section being approximately half thesum of the flare angle of said first flared section and the flare angleof the third section, said second flared section being so proportionedthat the flare angle change between said second flared section and saidthird section cancels substantially all of a mode, different from saiddominant mode and said one higher mode, produced at the flare anglechange between said first flared section and said second flared section.5. The means of claim 3 wherein said portion of TE/TM12 mode produced atsaid flare angle changes yields a ratio of TE/TM12 to TE10 of betweenabout 0.65 and about 0.84.
 6. Means for transmitting and receivingelectromagnetic wave energy comprising: a waveguide part of uniform,regular cross-section, said waveguide part being proportioned to supporta first mode in a transmit frequency band and in a receive frequencyband, and to support a second, higher, mode in one only of saidfrequency bands, a horn part of the same regular cross-section as saidwaveguide part connected to said waveguide part, means in said waveguidepart for converting a desired portion of said first mode of said one ofsaid frequency bands to said higher mode, said means in said waveguidepart having substantially no effect on the other of said frequencybands, and means in said horn part for converting a desired portion ofsaid first mode of the other of said frequency bands to a second,higher, mode, said means in said horn part comprising a plurality ofsmall flare angle changes at large cross-sectional dimensions of saidhorn part at least three times the cross-sectional dimension of thewaveguide part.
 7. Antenna means for transmitting and receivingelectromagnetic wave energy comprising: an input transmission line parthaving an output end, said transmission line comprising at least onewaveguide proportioned to propagate electromagnetic wave energy in adominant mode in at least a transmit frequency band and in a receivefrequency band, and a horn part connected to said waveguide part at oneend and having an aperture at its other end, first means in said hornpart for converting a desired portion of said first mode of one of saidfrequency bands to a second, higher, mode, said first means comprisingat least one small flare angle change at a large cross-sectionaldimension of said horn part at least three times the cross-sectionaldimension of the output end of the waveguide, said horn part beingproportioned to produce a desired phase relationship between said firstand second modes at said aperture, and second means at the output end ofthe transmission line part for converting a portion of said first modeof the other of said frequency bands to said higher mode, said secondmeans being located at a cross-sectional dimension of said antenna meanssubstantially smaller than said first means and having substantially noeffect on said one frequency band.
 8. Means for transmitting orreceiving a beam of electromagnetic wave energy comprising: an inputtransmission line comprising at least one waveguide proportioned topropagate electromagnetic wave energy in at least a first frequency bandin a dominant mode, a horn part connected to said transmission line,said horn part having at least four axial flare sections of differentflaring angles, one of said flare sections converting a desired portionof said dominant mode to a higher order mode and comprising a smallflare angle change in said horn at a large cross-sectional dimension ofsaid horn part at least three times the cross-sectional dimension of theinput waveguide, said horn part being proportioned to produce a desiredphase relationship between said dominant mode and said higher order modeinduced by said flare angle change.
 9. The means of claim 8 wherein saidsmall flare angle change induces said first-mentioned higher order modeand a second higher order mode, and including means, comprising at leasta second small flare angle change, proportioned and spaced from saidfirst-mentioned small flare angle change to cancel said second higherorder mode.
 10. Means for transmitting or receiving a beam ofelectromagnetic wave energy comprising: an input transmission linehaving an output end, said transmission line being proportioned topropagate electromagnetic wave energy in at least a first frequency bandin a dominant mode, and a horn of regular cross-section, said horncomprising a first section; a second, flared, section connected to saidfirst section to form a first flare angle change at a firstcross-sectional dimension of said horn; and a third section connected tosaid second section to form a second flare angle change at a secondcross-sectional dimension of said horn, the product of said first flareangle change times said first cross-sectional dimension beingapproximately equal to the product of the second flare angle changetimes said second cross-sectional dimension, said first flare changeinducing a first higher order mode and a second higher order mode insaid first frequency band, said second section being so proportionedthat said second flare angle change cancels substantially all of thesecond higher order mode in said first frequency band.